With advances in OLED technology, allowing several OLED devices to be stacked in a multi-device configuration, ‘colour-tuneable’ devices are becoming interesting. In a single-unit device, an active layer is sandwiched between a top electrode and a bottom electrode. The colour of the emitted light is largely determined by the composition of the active layer, which can comprise various organic polymers such as polyfluorenes chosen for their specific chemical structure that allows the colour of the emitted light to be determined. A single-unit device emits light of a certain colour, and that colour cannot be altered dynamically. A multi-device OLED, on the other hand, comprises two or more OLED devices or units ‘stacked’ on top of each other such that the anode of one unit is the cathode of the other unit in the stack. This shared or ‘inter’ electrode is effectively sandwiched between the different active layers of the multi-device OLED. The separate active layers can have different layer composition so that each OLED device or unit can have a different colour. The colour of the light emitted by the combined OLED devices can be tuned by regulating the current supplied to the electrodes, while an ‘inter’ electrode is used to drive one or both of its adjacent OLED devices. Obviously, the inter electrode must be at least partially transparent. A prior art colour-tuneable multi-device OLED comprising two stacked units is made by applying at least three isolated electrically conductive regions onto a substrate, which may be glass, polyethylene naphtalate, or some other suitable material, usually transparent since most devices are ‘bottom emitting’, i.e. they emit through the substrate. A transparent conductor is used for these conductive regions, for example doped zinc oxide, indium tin oxide or poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), usually referred to as Pedot:PSS. These conductive regions are applied such that a relatively large central anode region is flanked on one side by a contact pad for the inter electrode and on the other side by a contact pad for the cathode. A first active layer is applied onto the anode, and an inter electrode layer is applied to cover the first active layer, extending onto one of the contact pads. A second active layer is applied onto the inter electrode, and a third electrode is applied onto the second active layer, extending onto the other contact pad. For a bottom-emitting device, the third or top electrode can comprise a relatively thick layer of a highly reflective material such as aluminium or silver.
Because the three electrodes of such a prior art device can be addressed individually, the light emission of the two OLED units can be tuned relative to each other. With different organic materials for the two units, the light emitted by the top unit can have a slightly or even distinctly different colour from the light emitted by the bottom unit, allowing the combined colour or colour point of the combined device to be tuned simply by regulating the current applied to the electrodes.
However, the prior art design is associated with several drawbacks. The achievable device size is limited by the poor conductivity of the transparent anode. Also, since the inter electrode must be at least partially transparent, this layer must be very thin, with the result that its lateral conductivity is also inherently poor. The poor conductivity of the transparent electrodes effectively limits the device size to a maximum of about 5 cm by 5 cm. Another disadvantage is the inhomogeneous quality of the emitted light owing to the asymmetric contacting of the inter electrode and the cathode, so that the brightness of the light is uneven and drops off with increasing distance from the contact pads. Furthermore, the light-emitting area of the overall device is further restricted by the necessity for these contact pads, which occupy a significant portion of the available substrate area.
It is therefore an object of the invention to provide an improved multi-device OLED.